The invention concerns an electromagnet with a plunger. An electromagnet of this type is known from the German Patent No. 27 20 877 (GB 1571769) and serves preferably as the control of a hydraulic pressure control valve. The control device of the prior art, as well as of the electromagnet of the present invention serves the automatic adaptation of the magnetic force to a set value. The adaptation is to occur independently from the travel, i.e., of the position of the plunger within its stroke length. To that end, a measured value representing the current magnetic induction is transmitted to the control device, the measurement being performed by the sensor mentioned in the preamble of claim 1. The measured value and the set value are compared with each other in the control device; in case of a variation between the measured value and the set value, the control device automatically triggers a change of the excitation current, in such a way that the measured value will approach the set value.
In the prior electromagnet, the sensor element is arranged in the working air gap, that is, between the movable plunger and the fixed pole component. The advantage of this arrangement is constituted by the fact that the active area of the sensor element (which preferably is fashioned by a Hall generator) will be passed by the magnetic flow perpendicularly. The active area of the sensor element is the plane in which the charge carriers move, and this plane is located parallel to the working gap. Under these conditions, the induction measured in the gap has an optimum correlation to the magnetic force. Therefore, this prior arrangement of the sensor element provides optimum prerequisites for enabling the said control device to fulfill the purpose described above. However, a disadvantage of the prior arrangement of the sensor element is that it is located at a point where it is mechanically rather vulnerable and where, under certain conditions, it is exposed to an aggressive fluid ingressing from the pressure control valve.
An attempt at solving this problem is known from German Patent disclosure 36 05 216. It arranges the sensor element sideways and outside the interior space enveloped by the solenoid coil, and at that, in the area of that end face of the coil from which the plunger extends into the interior of the coil. The sensor element is located there in an area which is sealed against fluid access. However, this arrangement of the sensor element is associated with the disadvantage that not only the useful magnetic flux relevant for the onset of the magnetic force is measured but also a so-called stray flux, the magnitude of which depends on the current width of the air gap. Said stray flux decreases with a reduction of the air gap. Once the control device goes into action, this causes an undesirable contingency of the magnetic flux (and thus of the magnetic force) on the width of the air gap between the plunger and pole component.
Therefore, the problem underlying the invention is to improve the electromagnet known from the German Patent document 27 20 877 to the effect that the sensor element can be accommodated at a location which is safer than heretofore, and at that, without losing the previous advantage that the magnetic flux (and thereby the magnetic force) can be measured with high accuracy. Specifically sought is the avoidance of an adulteration of the measuring result by a so-called stray flux.
In other words, according to the invention, an annular gap subdivides the pole component in two pole component parts which essentially lie coaxially with one another and are magnetically insulated from each other. As a result, a rather exactly radial direction of the magnetic flux results in the annular gap, which now accommodates the sensor element. The latter is so inserted in the annular gap that its active surface lies parallel to the annular gap. As a result, the active surface of the sensor, in turn, is passed perpendicularly by the magnetic flux.
Furthermore, the sensor element (compared to the German Patent Disclosure 36 05 216) lies no longer in the area of that end of the solenoid coil from which the plunger extends into the interior of the coil. Instead, the sensor element now is located in the area of the opposite end of the solenoid coil, i.e., where the fixed pole component extends into the interior of the solenoid coil. All of these measures cause the sensor element (which preferably is fashioned as a Hall generator) to be passed exclusively (or nearly exclusively) by the useful magnetic flux, i.e., by the flux passing through the plunger. Thus, the sensor element is at least extensively free of interfering stray flux. At the same time, in contrast to the German Patent Document 27 20 877, it is located at an extremely well protected point. The risk of injury to the sensor element is now nearly zero. Moreover, the arrangement offers the advantage that the magnetic resistance of the annular gap (which accommodates the sensor element) remains relatively small, due to the rather large cylindrical surface of the annular gap.
Additionally, the sensor element can now be protected from fluids, specifically aggressive fluids. For that purpose, the annular gap will be sealed at the end of the pole component that faces toward the plunger with a nonmagnetic material. This is especially important when the solenoid is used to control a hydraulic pressure control valve and, thus, is installed directly on it.
The annular gap may have various shapes, for instance conical and/or with a shoulder. The cylindrical shape is preferred in order to simplify the manufacture. The clearance of the annular gap can vary across the length of the solenoid but is preferably made constant.
The effect described above, namely measuring on the sensor element the flux passing through the plunger, can be further improved by making the magnetic resistance in the fixed pole component on both sides of the annular gap at least approximately identical, provided the air gap (i.e., the distance between the plunger and the pole component) assumes a minimum value. This adaptation of the magnetic resistance in the two areas of the pole component can be effected in an especially simple way by providing in the plunger a ring-shaped recess in the end face facing toward the fixed pole component. The depth of the recess can be determined by trial or by computation. What can be accomplished thereby is that the magnetic force will be entirely independent of travel. Or, if desired, a specific contingency on travel of the magnetic force can be accomplished.
The above control and the pertaining components are preferably arranged (as known from the German Patent Document 27 20 877) between the outer end face of the pole component and the device to be controlled (for instance the pressure control valve) in a so-called electronic space. The annular space is suitably open toward the electronic space, at least where the sensor element is arranged. This greatly facilitates the assembly of the sensor element and of the pertaining electric lines.